Process for Treating a Loaded Solvent Stream Having a Time-Varying Concentration of Contaminant

ABSTRACT

The invention provides a process for treating a loaded solvent stream having a time-varying contaminant concentration, the process comprising the steps of: (a) providing a plurality of hold-up tanks; (h) feeding the loaded solvent stream in dependence on its contaminant concentration to one or more of the hold-up tanks and; (c) allowing lowed solvent to flow from the plurality of hold-up tanks to obtain a smoothed loaded solvent stream having a reduced time-varying contaminant concentration. The invention further provides a treating unit comprising a circuit for circulating a solvent stream, which circuit includes a device for smoothing contaminant peak concentrations, said device comprising a plurality of hold-up tanks each hold-up tank having at least one inlet and an outlet equipped with an outlet valve, the device further comprising an inlet distributor allowing the control of solvent flow to one or more of the hold-up tanks.

FIELD OF THE INVENTION

The invention relates to a process and treating unit for treating aloaded solvent stream having a time-varying concentration of acontaminant.

BACKGROUND OF THE INVENTION

Loaded solvent streams can be obtained from processes for removingcontaminants such as acidic compounds from a gas stream by washing thegas stream with fresh solvent. The use of solvents for removingcontaminants such as acidic gasses is well known, see for instance thebooks A. L. Kohl and F. C. Riesenfeld, 1974, Gas Purification, 2^(nd)edition, Gulf Publishing Co. Houston and R. N. Maddox, 1974, Gas andLiquid Sweetening, Campbell Petroleum Series.

Preferably, a regenerable solvent is used in a continuous process.

Generally, loaded solvent streams are treated by removing thecontaminant and regenerating the solvent The removal of the contaminantfrom the loaded solvent stream can be effected for example by contactingthe loaded solvent stream with a suitable stripping agent and removingthe contaminant from the loaded stripping agent. In the case where theconcentration of contaminant in the loaded solvent varies with time, theconcentration of the contaminant in the loaded stripping agent will alsovary in time. The contaminant removal from a loaded stripping agent thathas a time-varying concentration of contaminant can cause problems,especially when the conditions such as temperature, pressure, size ofcatalyst bed etc. required to remove such a contaminant depend to alarge extent on the concentration of the contaminant.

It is therefore desirable to provide a treating process enabling thefurther processing of loaded solvents with a time-varying concentrationof contaminant.

SUMMARY OF THE INVENTION

To this end, the invention provides a process for treating a loadedsolvent stream having a time-varying contaminant concentration, theprocess comprising the steps of: (a) providing a plurality of hold-uptanks; (b) feeding the loaded solvent stream in dependence on itscontaminant concentration to one or more of the hold-up tanks and; (c)allowing loaded solvent stream to flow from the plurality of hold-uptanks to obtain a smoothed loaded solvent stream having a reducedtime-varying contaminant concentration.

Step (b) and step (c) can be done in sequence but can also be doneparallel.

The invention further provides a treating unit comprising a circuit forcirculating a solvent stream, which circuit includes a device forsmoothing contaminant peak concentrations, said device comprising aplurality of hold-up tanks, each hold-up tank having at least one inletand an outlet equipped with an outlet valve, the device furthercomprising an inlet distributor allowing the control of solvent flow toone or more of the hold-up tanks.

The use of a plurality of hold-up tanks instead of for example a singlestirred tank offers advantages. In a single stirred tank, the loadedsolvent would be collected and stirred in the hold-up tank. A problemarising when using a single stirred tank is that measures have to betaken to ensure that the whole tank is stirred to achieve a homogeneousconcentration of the contaminant.

Another problem when using a single stirred tank is that the inlet peakof loaded solvent entering the single stirred tank would always cause anoutlet peak. Only in the case of an infinitely large tank could theoutlet peak be eliminated. When using a plurality of hold-up tanks, thesmoothing of peak concentrations of contaminant in the solvent streamcan be achieved. After treatment in a process according to theinvention, the concentration of contaminant in the treated solventstream shows less fluctuation in time.

The process and unit according to the invention ensure that the finaloutlet flow from the plurality of hold-up tanks, which is the combinedstream from the first and the second outlet streams, has a reducedtime-varying contaminant concentration.

DETAILED DESCRIPTION OF THE INVENTION

Smoothing of time-varying contaminant concentration in loaded solventstreams is desirable for optimum operation of a contaminant removalunit, wherein contaminants are removed from the loaded solvent stream.

A particular situation is when a loaded solvent stream is derived froman upstream unit for removal of contaminants from a gas stream. Thissituation occurs for example in a gas treating unit whereincontaminants, especially sulphur contaminants such as mercaptans and/orhydrogen sulphide, are removed from a loaded gas stream in an upstreamunit comprising at least one adsorbent bed. In the case where theupstream unit comprises two beds, one bed will be in adsorption mode(meaning that the operating conditions of the bed are such thatcontaminant adsorption from the loaded gas stream onto the bed will takeplace), while the other bed is in regeneration mode (meaning that theoperating conditions of the bed are such that contaminant desorptionwill take place from the bed into a regeneration gas stream). Becauseregeneration and thus, desorption of contaminants, usually takes placeduring a relatively short period of time, the concentration ofcontaminants in the regeneration gas will not be constant with time.Instead, there will be time intervals where the concentration ofcontaminants is relatively high compared to the average concentration ofcontaminant over a long period of time (peak concentrations) and timeintervals where the contaminant concentration is relatively low comparedto the average concentration of contaminant over a long period of time(off-peak concentrations).

In general, the regeneration gas stream having a time-varyingcontaminant concentration is purified by contacting the regeneration gasstream with a solvent, whereby the contaminants are transferred from theregeneration gas stream to a solvent. In this way, a solvent streamloaded with contaminants having a time-varying contaminant concentrationis obtained because the peak and off-peak contaminant concentrations inthe regeneration gas stream will translate into peak and off-peakcontaminant concentrations in the loaded solvent stream. In other words,the regeneration gas stream will have a time-varying contaminantconcentration.

When purifying the loaded solvent stream having a time-varyingcontaminant concentration, typically by contacting the solvent streamwith a suitable stripping agent, a stripping agent stream loaded withcontaminants is obtained. The contaminants are typically removed fromthis loaded stripping agent stream in a contaminant recovery unit.

In the contaminant recovery unit, contaminants are converted to aproduct suitable to be discharged from the contaminant recovery unit.Generally, the operating conditions of the contaminant recovery unit areset to enable converting a certain range of contaminant concentration inthe loaded stripping agent stream. If the loaded stripping agent streamcomprises a time-varying concentration of contaminant, the operatingconditions in the contaminant recovery unit will have to be adjustedeach time the contaminant concentration is outside this range, to enableconverting the contaminants to a desired level. This requires a morecomplicated contaminant recovery unit and results in a cumbersomeprocess.

The process according to the present invention enables the removal oftime-varying concentrations of a contaminant, resulting in a loaded gasstream with a constant concentration of contaminant. This offersadvantages over a conventional process wherein the time-varyingconcentration of a contaminant in the loaded solvent is not dealt with.

The invention will now be discussed in more detail by way of examplewith reference to FIG. 1.

FIG. 1 shows schematically a typical treating unit according to theinvention. The treating unit comprises a circuit for circulating asolvent stream, indicated by a rectangle that is referred to withreference number 1. The circuit includes an absorber unit indicated by arectangle that is referred to with reference number 2, a device forsmoothing contaminant peak concentrations indicated by a rectangle thatis referred to with reference number 3 and a regenerator unit indicatedby a rectangle that is referred to with reference number 4.

The absorber unit 2 includes an absorber 5 with outlets and inlets. Agas stream loaded with contaminants can enter the absorber via inlet 6.In the absorber, the gas stream loaded with contaminants is contactedwith regenerated solvent via line 21, whereby the contaminants aretransferred from the gas stream to the solvent, creating a loadedsolvent stream. The treated gas stream exits the absorber via line 7.The gas absorber comprises internals 8. Internals in an absorber whereina gas mixture is contacted with an absorbent solvent are known in theart and can comprise contacting layers, suitably from 5-80 contactinglayers, such as valve trays, bubble cap trays, baffles and the like.Structured packing may also be applied. One outlet of the absorber isconnected via line 9 to the device for smoothing contaminant peakconcentrations 3.

The device for smoothing contaminant peak concentrations comprises aninlet distributor 10, a first hold-up tank 11 and a second hold-up tank12. The inlet distributor 10 allows the solvent to flow either into thefirst hold-up tank 11 via inlet 13 or into the second hold-up tank 12via inlet 14.

A preferred suitable inlet distributor is a three-way valve (as shown inFIG. 1), designed in such a way that the flow of solvent is directed toonly one of the hold-up tanks during a time interval. This allowsdirecting the flow either to the first hold-up tank, or alternatively tothe second hold-up tank.

Another preferred inlet distributor is a system of two separate controlvalves (not shown in FIG. 1), one control valve connected to the firsthold-up tank and the other control valve connected to the second hold-uptank, the two control valves adjusted in such a way that when onecontrol valve allows the flow of solvent stream to enter one of thehold-up tanks during a certain time interval, the other control valveprevents the flow of solvent stream to enter the other hold-up tank.Alternatively, the two control valves can be adjusted in such a way toallow the flow of solvent to enter both tanks simultaneously. Preferablythe inlet distributor is arranged so as to direct the flow to the tanksin dependence on the concentration of the loaded solvent stream. Thiscan be based on knowledge on the time dependence of the contaminantconcentration, as explained hereinabove, and/or can be based oninformation gathered using means for determining the contaminantconcentration.

The first hold-up tank has an outlet 15 and the second hold-up tank hasan outlet 16. Both outlets are equipped with valves that can allow orprevent the solvent contained in the hold-up tank to exit the hold-uptank via the outlet. Both outlets 15 and 16 debouche into line 17. Line17 is connected to the regenerator unit 3.

The regenerator unit 3 comprises a regenerator 18 with inlets andoutlets. Line 17 debouches into an inlet of regenerator 4. Regeneratorsor stripping columns are well-known. The regenerator can optionallycomprise internals 19. In the regenerator, the loaded solvent stream isheated and contacted with a stripping agent stream, for example astripping gas stream, whereby the contaminants are transferred from thesolvent stream to the stripping agent stream, creating a loaded gasstream and a regenerated solvent. The loaded stripping agent stream canexit the regenerator via outlet 20. The regenerated solvent stream canbe led back to the absorber via line 21, thereby completing the circuit.

In the process according to the invention, the loaded solvent stream isfed in dependence to its contaminant concentration to one or more of thehold-up tanks. It can in general be advantageous to lead the loadedsolvent stream into either a first or a second tank. But in somesituations it may also be preferred to feed more than one tank at thesame time, for example depending on the filling level in the tanks.

It will be understood that the invention also includes systemscomprising more than 2 hold-up tanks. For example, the hold-up tank forpeak concentrations can be split up into two or more smaller tanks toenable easier mixing while at the same time the two or more smallertanks operate as one hold-up tank. Likewise, the hold-up tank foroff-peak concentrations can be split up into two or more smaller tankswhile at the same time the two or more smaller tanks operate as onehold-up tank. Another example is feeding the loaded solvent stream tothe plurality of hold-up tanks according to more than one contaminantconcentration threshold, for example: high contaminant concentration,intermediate contaminant concentration and low contaminantconcentration. It will be understood that the details of the processcontrol depend inter alia on the contaminants of the solvent streamand/or on the variation in contaminant concentration and can be adjustedto achieve the desired reduction in time-variation of contaminantconcentration in the treated solvent stream.

To enable feeding the loaded solvent stream to the tanks in dependenceto the contaminant concentration, knowledge about the time-variation ofthe contaminant concentration is needed.

In one embodiment, knowledge about the time-variation of the contaminantconcentration is known in advance because it is derived from upstreamprocess steps. For example, as explained earlier, the peak and off-peakcontaminant concentrations in a regeneration gas stream will translateinto peak and off-peak contaminant concentrations in the loaded solventstream. Thus, in the situation where the loaded solvent stream isobtained from an upstream process, the time variation in contaminantconcentration in the loaded solvent stream can be derived from thetime-variation of contaminant concentration in the upstream process.This information is generally sufficient for feeding the loaded solventstream to the hold-up tanks in dependence on contaminant concentrationand it is then not necessary to measure contaminant concentration.

Therefore, in a preferred embodiment, the invention is a process fortreating a loaded solvent stream having a time-varying concentration ofa contaminant, the process comprising the steps of: (a) leading at leastpart of the loaded solvent stream to a first hold-up tank during a timeinterval t1, the first hold-up tank issuing a first outlet solventstream; (b) leading at least part of the loaded solvent stream to asecond hold-up tank during a time interval t2, the second hold-up tankissuing a second outlet solvent stream; and (c) combining the first andsecond outlet solvent streams issued from the hold-up tanks to a finaloutlet solvent stream.

In a specific embodiment, the invention is a process for treating aloaded solvent stream having a time-varying concentration of acontaminant, the process comprising the steps of: (a) leading the loadedsolvent stream to a first hold-up tank during a time interval t1, thefirst hold-up tank issuing a first outlet solvent stream; (b) leadingthe loaded solvent stream to a second hold-up tank during a timeinterval t2, the second hold-up tank issuing a second outlet solventstream; and (c) combining the first and second outlet solvent streamsissued from the hold-up tanks to a final outlet solvent stream.

Time interval t1 (peak concentrations) and time interval t2 (off-peakconcentrations) refer respectively to the time interval during which theconcentration of contaminant is relatively high compared to the averageconcentration of contaminant over a long period of time (t1) and thetime interval during which the concentration of contaminant isrelatively low compared to the average concentration of contaminant overa long period of time (t2).

The process is especially advantageous when the first outlet flow is(t1/T)*circulation rate of the solvent, and the second outlet flow is(t2/T)*circulation rate of the solvent, T being the run cycle timedefined as the sum of t1 and t2.

As set out earlier, the loaded solvent stream having peak and off-peakconcentrations can originate from an upstream unit comprising at leasttwo adsorbent beds. The run cycle time T will then be the shortest timeinterval (period) after which the contaminant concentration versus timefunction of the regeneration gas stream repeats itself.

Fluctuation in contaminant concentration can be especially cumbersome incases where the technology used for removal of contaminant is sensitiveto fluctuations in concentration. Examples are removal of mercaptans orhydrogen sulphide.

In cases where it is desirable to reduce fluctuations as much aspossible, it is desirable to maintain a multiplication factor in therange of from 1.0 to 1.7. In cases where a certain degree of fluctuationis acceptable, the multiplication factor can be in the range of from 0.3to 0.95, preferably from 0.4 to 0.9. This allows the use of smallerhold-up tanks, giving advantages in terms of setting-up and running thegas-treating unit, as well as economical benefits, while at the sametime the extent of reduction in the fluctuation of contaminantconcentration is still sufficient.

In a preferred embodiment, t1 and t2 are known in advance orpredetermined. For example, when the loaded solvent stream is obtainedfrom an upstream process wherein contaminants are transferred from a gasstream to a solvent stream to obtain the loaded solvent stream, thetime-dependency of the contaminant concentration (and thus, t1 and t2)can be known or predetermined from the time-dependency of contaminantconcentration in the gas stream. An advantage of this embodiment is thatthe size of the hold-up tanks can be dimensioned such that apredetermined concentration fluctuation of contaminant in the smoothedsolvent stream can be guaranteed, without the need for additionalequipment for measuring contaminant concentration. In the event that thetime variation of the contaminant concentration is known in advance,this knowledge can be used at the design stage of the device forsmoothing contaminant peak concentrations, in particular for selectingthe volume of the hold-up tanks. The volume of the first hold-up tank issuitably approximately equal to the circulation rate of the solvent (inm³/s) through the circuit, multiplied by a first time interval t1 (in s)and multiplied by a factor in the range of from 0.3 to 1.7. The volumeof the second hold-up tank is approximately equal to the circulationrate of the solvent (in m³/s) through the circuit multiplied by a secondtime interval t2 (in s) and multiplied by a factor in the range of from0.3 to 1.7. The volume of the first and second hold-up tank can beadjusted in order to achieve the desired degree of reduction offluctuation in contaminant concentration.

The value of t1 is typically in the range of from 0.05 to 0.9 times T,preferably from 0.1 to 0.7 times T, more preferably from 0.2 to 0.5times T, still more preferably from 0.3 to 0.4 times T. The total runtime of the process comprises several run cycles. It will be understoodthat one run cycle can comprise several intervals wherein theconcentration of contaminant is relatively high and several timeintervals wherein the concentration of contaminant is relatively low.

In the case where one run cycle comprises several intervals wherein theconcentration of contaminant is relatively high and several timeintervals wherein the concentration of contaminant is relatively low, t1refers to the accumulated time intervals wherein the concentration ofcontaminant is relatively high and t2 refers to the accumulated timeintervals wherein the concentration of contaminant is relatively low.

Typically, during t1 the contaminant concentration in the loaded solventstream is between about 5% to about 10% higher, relative to the averageconcentration measured during the sum of both time intervals t1 and t2.

Typical values for t1 range from about 0.05 and 0.9 times the totalcycle time T, preferably between 0.1 and 0.7 times T, more preferablybetween 0.2 and 0.5 times T, still more preferably between 0.3 and 0.4times T.

In the event that the time-variation of the contaminant concentration isnot known in advance, a threshold value can be predetermined orpredefined. The loaded solvent stream can then be allowed to flow intoone hold-up tank if the concentration of the contaminant exceeds thispredefined threshold value and to another hold-up tank if thecontaminant concentration is below the predefined threshold value. Itwill be understood that the threshold value can be adapted over time orthat there can be more than one predetermined or predefined thresholdvalue, to enable several degrees of reduction in time-variation ofcontaminant or to take into account the presence of more than onecontaminant.

Threshold values depend inter alia on the nature and type of thecontaminant to be removed. For typical contaminants, for example sulphurcontaminants such as mercaptans and/or hydrogen sulphide, thresholdvalues range from about 2 ppmv to about 1 volume %, preferably fromabout 5 ppmv to about 0.5 volume %, calculated as the concentration ofcontaminant in the loaded solvent. Typical threshold values formercaptans (RSH) are from about 3 ppmv and about 15 ppmv.

One way to determine whether the contaminant concentration is above thepredefined or predetermined threshold value is to measure thecontaminant concentration. The contaminant concentration can for examplebe monitored online using a suitable analysis method and based on theresult, the loaded solvent flow is then led to one or more hold-uptanks. However, it may not be possible to exactly dimension the hold-uptanks such that a predetermined reduction in time-varying contaminantconcentration in the outlet flow to the contaminant recovery unit can beguaranteed under all circumstances. Still, even in this embodiment aconsiderable reduction in time-variation of contaminant concentrationcan be achieved.

A further way to take a concentration measurement into account infeeding the loaded solvent stream to the tanks is the following. Thecontaminant concentration is monitored over a certain period of time toevaluate whether the time-variation shows a pattern in time, for exampleperiods of relatively high contaminant concentration, alternated byperiods of relatively low contaminant concentration. Based on thispre-established time-dependency, the control of the flow to the hold-uptanks can be performed.

The process according to the invention enables the treatment of anyloaded solvent stream having a time-varying concentration of acontaminant. Reference herein to a contaminant is to one or morecompounds to be removed from the solvent stream. Typically, thecontaminant is an acidic compound, for example a sulphur compound.

In a preferred embodiment, the contaminant is one or more compoundsselected from the group of H₂S, RSH, CS₂ and COS, preferably RSH.Especially the removal of H₂S and mercaptans (RSH) from a gas streamcomprising these compounds is of considerable importance, because thetoxicity and smell of H₂S and mercaptans renders their presence highlyundesirable. Moreover, H₂S is very corrosive to the gas pipelinenetwork. In view of the increasingly stringent environmentalrequirements, removal of H₂S and mercaptans has become even moreimportant.

Reference herein to RSH is to aliphatic mercaptans, especially. C₁-C₆mercaptans, more especially C₁-C₄ mercaptans, aromatic mercaptans,especially phenyl mercaptan, or mixtures of aliphatic and aromaticmercaptans. The invention especially relates to the removal of methylmercaptan, ethyl mercaptan, normal- and iso-propyl mercaptan and butylmercaptan isomers.

Suitable solvent streams are solvent streams comprising physicalsolvents or solvent streams comprising a combination of chemical andphysical solvents. Suitable chemical solvents are primary, secondaryand/or tertiary amines derived alkanolamines or aqueous solutionsthereof. The most frequently used amines are derived from ethanolamine,especially monoethanol amine (MEA), diethanolamine (DEA),triethanolamine (TEA), diisopropanolamine (DIPA) andmethyldiethanolamine (MDEA). Suitable physical solvents are selectedfrom the group of cyclo-tetramethylene-sulfone and its derivatives,aliphatic acid amides, N-methylpyrrolidone, N-alkylated pyrrolidones andthe corresponding piperidones, methanol, ethanol, mixtures ofdimethylethers of polyethylene glycols, mixtures of dialkylethers ofpolyethylene glycols and N-methyl-2-pyrrolidone (NMP).

A preferred solvent stream is a solvent stream comprising an aqueousmixture of a chemical solvent, especially DIPA and/or MDEA, and aphysical solvent, especially cyclotetramethylene-sulfone. Such a mixtureshows good absorption capacity and good selectivity against moderateinvestment costs and operational costs. It performs very well at highpressures, especially between 20 and 90 bara.

Another preferred solvent stream is a solvent stream comprisingN-methyl-2-pyrrolidone (NMP). NMP is a high boiling solvent which has anespecially high solubility for H₂S. This solvent is especially suitablefor selective H₂S absorption in the presence of CO₂.

Yet another preferred solvent stream is a solvent stream comprisingmixtures of dimethylethers of polyethylene glycols and/or mixtures ofdialkylethers of polyethylene glycols. The solubility of H₂S in thesesolvents is high, enabling the selective removal of H₂S. Furthermore,mercaptans, especially methyl mercaptan, are highly soluble in mixturesof dimethylethers of polyethylene glycols, enabling the selectiveremoval of mercaptans.

The amount of contaminant removal in the absorber unit can be optimisedby regulating the solvent/gas ratio. A suitable solvent/gas ratio isfrom 1.0 to 10 (w/w), preferably between 2 and 6.

The loaded solvent may contain beside contaminants such as CO₂, H₂S, RSHand/or COS also appreciable amounts of other compounds from the gasmixture to be purified, e.g. hydrocarbons, carbon monoxide, hydrogenetc. It may be advantageous to remove these (non-acidic) compounds atleast partially from the loaded solvent by flashing to a pressure whichis higher that the sum of the partial pressures of the contaminants. Inthis way only very small amounts of contaminants are released from thesolvent together with the (non-acidic) compounds. The loaded solvent mayadvantageously flashed in a second step to a pressure which is below thepartial pressures of the contaminants at the prevailing temperature.Usually the flash is carried out at a pressure between 1 and 15 bara,preferably between 1 and 10 bara, more preferably ambient pressure.

Typically, the hold-up tanks are kept at pressures between about 1 and150 bara, suitably between 2 and 100 bara, preferably between about 3and 15 bara. Typical temperatures at which the hold-up tanks are keptare from 0 to 130° C., preferably between 30 and 70° C., more preferablybetween 40 and 60° C.

In the regenerator unit, the regeneration can be done either at hightemperatures and ambient pressure or at high pressure and ambienttemperature. Typically, the regeneration unit is kept at a highertemperature than the absorber unit.

In a preferred embodiment, the loaded solvent, optionally after flashingas described above is regenerated by heating in a regeneration unit,suitably at a temperature between 30 and 170° C., preferably between 70and 150° C., suitably at a pressure between 1 and 2 bara. The heating ispreferably carried out with steam or hot oil.

In another preferred embodiment, the loaded solvent, optionally afterflashing as described above is regenerated at a relatively highpressure, preferably between 2 and 120 bara, more preferably between 5and 100 bara, suitably at a temperature between 10 and 30° C. The leanabsorbent solvent, issued from the regeneration unit, will be used againin the absorption unit as described before. Suitably the lean solvent isheat exchanged with the loaded solvent.

In an especially preferred embodiment, the loaded gas stream obtainedfrom the regeneration unit is led to a recovery unit wherein thecontaminant is removed from the stripping agent stream.

Suitably, the loaded solvent stream is obtained by the steps of: (d)introducing lean solvent into an absorber unit; (e) introducing acontaminated gas stream in the absorber unit; (f) contacting thecontaminated gas stream with the lean solvent in the absorber unit,thereby transferring the contaminant to the solvent to obtain treatedgas and the loaded solvent stream. The contaminated gas stream can beany inert gas stream comprising contaminants and may comprise synthesisgas, obtained for instance by (catalytic) partial oxidation and/or bysteam methane reforming of hydrocarbons, e.g. methane, natural orassociated gas, naphtha, diesel and liquid residual fractions, gasesoriginating from coal gasification, coke oven gases, refinery gases,hydrogen and hydrogen-containing gases, synthesis gas or natural gas.

In an especially preferred embodiment, the contaminated gas streamentering the absorber unit is the regeneration gas stream issued from amol sieve bed which is being regenerated, the mol sieve bed having beenused to absorb contaminants from a feed gas stream. Typically, the molsieve bed is regenerated either by increasing the temperature at ambientpressure or by increasing the pressure at ambient temperature. As setout earlier, when contaminants are removed from the regeneration gasstream in a process whereby the contaminants are transferred from theregeneration gas stream to a solvent, a solvent stream loaded withcontaminants is obtained. The peak and off-peak contaminantconcentrations in the regeneration gas stream will translate into peakand off-peak contaminant concentrations in the loaded solvent stream.

Optionally, the process can have the additional steps of: (g) contactingthe smoothed loaded solvent stream of step (c) in a regenerator unitwith a stripping agent stream to obtain a loaded stripping agent streamand a lean solvent stream and (h) leading the loaded stripping agentstream to a contaminant recovery unit. The lean solvent stream can thenbe led back to the absorber unit (step (i)).

By contacting the smoothed loaded solvent stream with a stripping agentstream, contaminants are transferred from the loaded solvent stream tothe stripping agent stream, resulting in a stripping agent stream loadedwith contaminants. The contaminants are typically removed from thisstripping agent stream in a contaminant recovery unit.

In the case of sulphur contaminants, the recovery unit is a sulphurrecovery unit. Typically, in the sulphur recovery unit the sulphurcontaminants are removed by catalytically converting them to elementalsulphur for example using the known Claus process. If the loaded gasstream comprises a time-varying concentration of sulphur contaminant,the operating conditions in the sulphur recovery unit will have to beadjusted to enable converting the sulphur contaminants. This requires amore complicated sulphur recovery unit and results in a cumbersomeprocess. The process according to the present invention enables theremoval of time-varying concentrations of a contaminant, resulting in aloaded stripping agent stream with a constant concentration ofcontaminant. This offers advantages over a conventional process whereinthe time-varying concentration of a contaminant in the loaded solvent isnot dealt with, resulting in a loaded stripping agent stream having atime-varying concentration of contaminant.

In a typical process according to the invention will now be describedwith reference to the figure. A contaminated gas stream having atime-varying concentration of contaminant is led via line 6 to absorber5. In absorber 5, the contaminated gas stream is contacted in a countercurrent way with a solvent, whereby the contaminant is transferred fromthe gas stream to the solvent.

A treated gas stream from which the contaminants have been removed isled from the absorber via line 7. A loaded solvent stream having atime-varying concentration of contaminant is issued from the absorberunit and led via line 9 to the device for smoothing contaminant peakconcentrations 3.

During time interval t1 the concentration of contaminant is relativelyhigh (peak concentrations) and the inlet distributor 10 is adjusted insuch a way that it connects line 9 to inlet 13 of the first hold-up tank11, allowing the loaded solvent stream to be collected in the firsthold-up tank 11. At the end of time interval t1, the inlet distributor10 is adjusted in such a way that it now connects line 9 to the inlet 14of the second hold-up tank 12, allowing the loaded solvent stream to becollected in the second hold-up tank 12 during time interval t2 duringwhich the concentration of contaminant is relatively low (off-peakconcentrations). The outlet 15 of the first hold-up tank 11 is equippedwith a first outlet valve which enables to regulate the outlet flow fromthe first hold-up tank in such a way that the outlet flow from the firsthold-up tank is (t1/T) *circulation rate of the solvent, T being the runcycle time defined as the sum of t1 and t2. In the regenerator, theloaded solvent stream is heated and contacted with a stripping gasstream, whereby the contaminants are transferred from the solvent streamto the gas stream, creating a loaded gas stream and a regeneratedsolvent stream. The regenerated solvent stream, which now containslittle or no contaminants, is led via line 21 back to the absorber 5.The loaded gas stream, which contains the contaminants, is led from theregenerator via line 20. In a preferred embodiment, line 20 is connectedto a recovery unit.

The invention will now be illustrated with the following, non-limitingexample.

At a solvent circulation rate of 5 m³/minute, a time t1 of 90 minutesand a cycle time T of 240 minutes, a system of two hold-up tanks, thefirst hold-up tank having a volume of 450 m³, the second hold-up tankhaving a volume 750 m³, results in a final outlet solvent stream whereinthe variation in concentration of contaminant is less than 0.05% (basedon the starting variation). The term “variation” as used herein refersto the difference in contaminant concentration between the maximum ofthe peak concentration and maximum of the off-peak concentration. Thetotal volume of the two hold-up tanks is 1200 m³.

When using a single continuously stirred tank with a volume of 1200 m³instead of the system of two hold-up tanks, a variation in concentrationof contaminant of 20% in the final outlet solvent stream is observed.Even when using a single continuously stirred tank with a volume of 2500m³, still a variation in concentration of contaminant of 10% (based onthe starting variation) in the final outlet solvent stream is observed.In order to further reduce the variation in concentration of contaminantin the final outlet solvent stream to approach the 0 to 0.05% range, thesize of the single stirred tank would have to be increased in such a waythat the limit of an infinitely large single stirred tank is approached.

The process and gas-treating unit according to the invention enables areduction in the variation in concentration of contaminant to less than0.05%. (based on the starting variation). In addition, a substantiallylower total volume of hold-up tanks is required compared to a processand gas-treating unit using a single stirred tank.

1. A process for treating a loaded solvent stream having a time-varyingcontaminant concentration, the process comprising the steps of: (a)providing a plurality of hold-up tanks; (b) feeding the loaded solventstream in dependence on its contaminant concentration to one or more ofthe hold-up tanks and; (c) allowing loaded solvent to flow from theplurality of hold-up tanks to obtain a smoothed loaded solvent streamhaving a reduced time-varying contaminant concentration.
 2. A processaccording to claim 1, wherein the loaded solvent stream is led to afirst hold-up tank during a time interval t1, the first hold-up tankissuing a first outlet solvent stream, wherein the loaded solvent streamis led to a second hold-up tank during a time interval t2, the secondhold-up tank issuing a second outlet solvent stream, and wherein thefirst and second outlet streams issued from the hold-up tanks arecombined to a final outlet stream which forms the smoothed loadedsolvent stream.
 3. The process according to claim 2, wherein t1 is atime interval during which the contaminant concentration exceeds athreshold value and t2 is a time interval during which the contaminantconcentration does not exceed the threshold value.
 4. A processaccording to claim 3, wherein the solvent stream withdrawn from thefirst hold-up tank is (t1/T)*circulation rate of the solvent, and thesolvent stream withdrawn from the second hold-up tank is(t2/T)*circulation rate of the solvent, T being the run cycle time.
 5. Aprocess according to claim 4, wherein the sum of t1 and t2 equals therun cycle time T, and wherein t1 has a value in the range of from 0.05to 0.9 times T.
 6. A process according to claim 5, wherein the variationof contaminant concentration in the loaded solvent stream with time isknown, and wherein feeding the loaded solvent stream in dependence onits contaminant concentration to one or more of the hold-up tanks isdone by feeding according to this knowledge about the time variation. 7.A process according to claim 6, wherein the variation of contaminantconcentration in the loaded solvent stream with time is measured, andwherein feeding the loaded solvent stream in dependence on itscontaminant concentration to one or more of the hold-up tanks is done bytaking this measurement into account.
 8. A process according to claim 7,wherein the loaded solvent stream is obtained by the steps of: (d)introducing lean solvent into an absorber unit; (e) introducing acontaminated gas stream in the absorber unit; (f) contacting thecontaminated gas stream with the lean solvent in the absorber unit,thereby transferring the contaminant to the solvent to obtain treatedgas and the loaded solvent stream.
 9. A process according to claim 8,further comprising the steps of: (g) contacting the smoothed loadedsolvent stream of step (c) in a regenerator unit with a stripping agentstream to obtain a loaded stripping agent stream, and a lean solventstream; (h) leading the loaded stripping agent stream to a contaminantrecovery unit.
 10. A process according to claim 9, wherein thecontaminant is a sulphur compound.
 11. A process according to claim 10,wherein the contaminant comprises one or more compounds selected fromthe group of H₂S, RSH and COS.
 12. A process according to claim 9,wherein the lean solvent which is introduced into the absorber unit instep (d) is the lean solvent stream obtained after step (g).
 13. Atreating unit comprising a circuit for circulating a solvent stream,which circuit includes a device for smoothing contaminant peakconcentrations, said device comprising a plurality of hold-up tanks eachhold-up tank having at least one inlet and an outlet equipped with anoutlet valve, the device further comprising an inlet distributorallowing the control of solvent flow to one or more of the hold-uptanks.
 14. A treating unit according to claim 13, wherein the volume ofthe first hold-up tank is approximately equal to the circulation rate ofthe solvent (in m³/s) multiplied by a first time interval t1 (in s) andmultiplied by a factor in the range of from 0.3 to 1.7 and the volume ofthe second hold-up tank is approximately equal to the circulation rateof the solvent (in m³/s) multiplied by a time interval second t2 (in s)and multiplied by a factor in the range of from 0.3 to 1.7.
 15. Atreating unit according to claim 14, wherein the sum of t1 and t2 equalsthe run cycle time T, t1 having a value of between 0.05 and 0.9 times T.16. A treating unit according to claim 15, further comprising anabsorber unit, the absorber unit including an absorber with inlets andoutlets, one outlet of the absorber connected to the inlet distributor.17. A gas treating unit according to claim 16, further comprising aregenerator unit, the regenerator unit including a regenerator withinlets and outlets, one outlet of the regenerator connected to theabsorber, one inlet of the regenerator connected to the inletdistributor.